FIG. 1 shows a commercially available prior art system for controlling flow. It uses a venturi meter 10, which has a input 81 and a throat 89 that is narrower than the input. The venturi meter 10 has a circular cross-section and is formed by inserting into a length of pipe, a short pipe that has an outer diameter equal to the inner diameter of the longer pipe. After the short pipe is inserted into the longer pipe, a hole is drilled through both pipes to provide a pressure conduction port 22 at the throat 89. At a point upstream of the this pressure conduction port 22, another hole is drilled only through the longer pipe to provide another pressure conduction port 21. Downstream of the venturi meter 10 is located a butterfly valve 19, which is controlled by a stepper motor. A microprocessor controls the motor so as to vary the amount the butterfly valve 19 impedes the flow based on the pressure differential between the two ports 21 and 22.